1. Field of the Invention
The present invention relates to a control system for an ultrasonic motor which controls rotation speed and rotational position of an output shaft of an ultrasonic motor.
2. Description of the Related Art
An electric motor utilizing electromagnetic force has been used widely in various industrial fields, and miniaturization thereof has been also carried out. However, since an armature including a winding is required in any type of electric motor, the use is restricted sometimes because of the large size and the like.
An ultrasonic motor compact in size and capable of generating a high torque and having good rotation controllability has been developed recently and already put into practical use (for example, see the specification of U.S. Pat. No. 4,562,374). Putting the driving principle in brief, this new ultrasonic motor, being different from an old type ultrasonic motor using a Langevin type oscillator, utilizes flexing oscillation having a progressive wave of a elastic ring-like member with piezoelectric elements stuck on one surface thereof, and drives by friction force a rotor which is in contact under pressure with the elastic member. In the surface of the elastic ring-like member with which the rotor is in contact, there are produced elliptic motions which rotate in a direction reverse to the propagating direction of the wave. Since the rotor is driven by friction force by means of very small displacement due to such motion, movement of the rotor, viz., rotation thereof, is stopped without delay when above-mentioned movement, viz., flexing vibration of the elastic ring-like member is stopped, thus generating no inertial rotation.
In order to make the displacement by elliptic motion in the surface of above-mentioned elastic ring-like member suitable for actual use, high frequency voltages are applied to piezoelectric elements so as to oscillate at a resonance frequency of a system including the elastic ring-like member or a frequency close thereto.
Since such a progressive wave type ultrasonic motor is small in size and produces a high torque, and further, vary low speed rotation is directly obtainable, it is effectual as a rotation drive means for an optical system of an apparatus performing circumferential welding in a very thin tube such as the laser beam welding apparatus for an inner circumferential surface of a tube disclosed in the specification of U.S. Pat. No. 4,839,495.
In such a circumferential welding apparatus utilizing a laser beam, it is only required to have a laser optical system make a single rotation. In order to perform welding of high quality, however, it is required to always maintain a scanning speed of a laser beam viz., circumferential rotating speed of the laser optical system constant.
In order to drive a progressive wave type ultrasonic motor, high frequency voltages having different phases such as voltages of a sine wave form and a cosine wave are applied to a elastic ring-like member, viz., piezoelectric element of a stator. A voltage of a resonance frequency is applied to an ultrasonic motor driver which generate two-phase alternating voltages. Accordingly, a frequency stabilizing circuit for generating a resonance frequency stably and accurately is desired in order to obtain a stabilized circumferential rotating speed.
In case a signal of a desired frequency is required to be outputted, it has been known in general to get it by dividing output pulses of a crystal oscillator. However, there has been such a problem that, in production of a signal of a high frequency (several hundred KHz for instance), decrease in resolution cannot be avoided and a stabilized frequency is unobtainable.
In another system, a digital frequency set value (a target frequency value) is converted into an analog voltage by means of a digital-analog (D/A) converter, and the analog voltage is converted into the signal of a desired frequency. FIG. 9 shows an example of such a system. In FIG. 9, a digital frequency set value received from a CPU (central processing unit) 40 is converted into an analog voltage by means of a D/A converter 41. This analog voltage is converted into an AC signal having a frequency set by a voltage/frequency (V/F) converter 42.
In a system combining the D/A converter and the V/F converter, however, there are such problems that analog adjustment for having an output frequency coincide with a digital frequency set value is required, and further, the output frequency may be varied depending on changes in temperature and the like. Therefore, conventional frequency output apparatuses have been unable to meet the necessity for a stable and accurate resonance frequency as described above.
As described, in order to obtain a stable circumferential rotating speed in an ultrasonic motor, it is required to apply a voltage of a stabilized frequency to an ultrasonic motor driver. An application of the voltage of a stabilized frequency is, however, insufficient for getting a stable rotating speed. It is required to pick up an actual rotating speed of the rotor of the ultrasonic motor and feed it back to the ultrasonic motor driver.
In order to control the rotating speed of an ultrasonic motor, it has been proposed that an output pulse signal of a rotary encoder coupled with the output shaft of the ultrasonic motor is converted from a frequency signal into a voltage signal (F/V conversion) so as to be fed back, and to apply pulse width modulation to the feed back signal. In such a pulse width modulation (PWM) system, however, the pulse width does not change before the rotation of the motor is decreased or increased. As a result, the rotating speed (same as the circumferential rotating speed) is not completely stabilized when load variation occurs. Further, in case adjustment of a loop gain for feedback is made inappropriately, the speed pulsates or the following-up time takes too long. Thus, a new control system which is capable of controlling the rotating speed of the ultrasonic motor stably has been desired.
Furthermore, accurate positioning is required in controlling the rotation of the ultrasonic motor. As a general method therefor, it has been known that output pulses of a rotary encoder coupled with the output shaft are counted and monitored, and the motor driving input is stopped when the count value reaches a value showing a target position. When such positioning control is made utilizing an electronic computer, there is such a problem that stop operation is delayed by the portion of scan time for monitoring the count of encoder pulses, thus deteriorating positioning accuracy.